1. Field of the Invention
The present invention relates to an exhaust gas treatment system for an internal combustion engine, particularly a diesel engine, including a catalyst arrangement.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In such exhaust gas treatment systems, the exhaust gas leaving an internal combustion engine is substantially purified by the catalyst arrangement from the pollutants which are particularly damaging to the environment. This exhaust gas contains, in particular, high proportions of nitrogen oxides and carbon monoxide, which are substantially converted by a catalytic reaction into carbon dioxide. Particularly in diesel internal combustion engines, soot particles also arise in such a catalytic reaction or in the combustion.
In order to start or maintain this catalytic reaction, a temperature is required which lies in the region of at least 200–250° C., according to the catalyst construction and material. If the catalyst temperature falls below a so-called “light-off temperature” situated in this region, it can no longer maintain, or respectively start, a catalytic reaction suitable for the required pollutant reduction. Such a fall in temperature in the catalyst region can, for example, arise when the exhaust gases leaving the internal combustion engine do not transport sufficient heat to be able to ensure a corresponding heating of the catalyst. This can be the case, for example, in idling operation, in which the temperature of the exhaust gases is in the region of 150° C. Particularly critical as regards a suitable catalytic reaction are therefore the starting phase of an internal combustion engine or also phases during travel in which travel is with low load, thus, for example, in lengthy downhill travel or when moving slowly on motorways, etc.
In order to prevent the soot particles transported in the exhaust gas of a diesel engine being ejected into the environment, efforts are being made to follow the catalysts with particle filters which can substantially filter out these soot particles from the exhaust gases ejected to the outside. However, the problem here is substantially that during the operating period these particle filters become choked with soot. It is therefore necessary to undertake a cleaning or regeneration from time to time. This can be performed, for example, by oxidation with nitrogen dioxide at a corresponding temperature, or by supplying additives and corresponding heating. However, the problem exists with all of these heretofore known cleaning methods that comparatively high temperatures of up to 400° C. are required, in order to ensure a suitable cleaning of the particle filter from the soot particles collected there. However, if the exhaust gas temperature of an internal combustion engine falls, or if this temperature is not high enough to start or maintain the said catalytic reaction, then lacking an ongoing catalytic reaction in the catalyst, also sufficient heating does not arise for so strongly heating the exhaust gases flowing toward the particle filter that the desired regeneration can be performed in the particle filter.